Sex and strain differences in morphine-induced temperature effects in WKYs and SHRs

Male and female normotensive Wistar-Kyoto rats (WKYs) and spontaneously hypertensive rats (SHRs) all responded to morphine treatment with biphasic dose-response curves, exhibiting hyperthermia at low doses and hypothermia at higher doses. However the direction and magnitude of temperature changes induced by different doses of morphine varied significantly depending upon the sex and strain (WKY vs. SHR) of the test animals. Both WKY and SHR males responded with little change in temperature at 1.0 mg/kg and hyperthermia at 5.0 mg/kg. Hypothermia appeared in SHR males at 10 mg/kg, while hypothermia in WKY males was not seen until 20 mg/kg was administered. Both WKY and SHR females demonstrated a greater sensitivity than their male counterparts to the thermotropic effects of morphine, exhibiting hyperthermia at 1.0 mg/kg, which was greater than the hyperthermia exhibited by male rats, and progressively greater hypothermia at 2.0, 5.0 and 10 mg/kg. SHR females demonstrated hypothermia at lower doses of morphine than did WKY females but were otherwise not different. These findings indicate that (1) morphine-induced temperature effects in SHRs and WKYs are dependent upon dose; (2) SHRs seem more sensitive than WKYs to the hypothermic effects of morphine; and (3) female rats seem more sensitive than male rats to the thermotropic effects of morphine in general.